Size your battery bank, solar panels, and charge controller for a van, cabin, or off-grid home. Shows losses at each stage and compares battery chemistries.
Add each appliance you use, its wattage, and hours per day.
Total daily load
0 Wh
With autonomy days
0 Wh
Solar panels (100W)
100W
0%
MPPT controller loss (3%)
97W
-3%
Battery charging (no loss)
97W
0%
Inverter loss (10%)
87W
-10%
Delivered to load
87W
-13% total
This example shows why you need more solar than your load suggests. To deliver 1000Wh daily to your appliances, you need roughly 1150Wh of panel input. Add 30-50% more for cloudy days and charging during autonomy periods.
Usable capacity needed
0 Wh
Total nominal capacity
0 Wh
Amp-hours at 12V
0 Ah
Lead-Acid Example
200Ah at 12V:2400Wh nominal
At 50% DoD:1200Wh usable
Weight (4 batteries):approx. 800 lbs
Lifespan:3-5 years
Replacement cost:approx. $1200
LiFePO4 Example
200Ah at 12V:2400Wh nominal
At 80% DoD:1920Wh usable
Weight (1 battery):approx. 150 lbs
Lifespan:10+ years
Replacement cost:approx. $3200
Total wattage needed
0 W
For cloudy days (buffer)
0 W
MPPT charge controller
0 A
Typical appliance wattages
Use these figures to estimate the power consumption of common van and off-grid appliances. Real wattage varies by model and efficiency rating.
Appliance
Power
Typical use
LED light strip (12V)
Low power if wired directly to battery.
5-20W
4-8 hrs
LED light (AC, through inverter)
60W equivalent LED.
9-12W
5 hrs
Water heater (12V demand)
Tankless or on-demand model.
1000-1500W
0.5 hrs
Refrigerator (12V compressor)
VanLife fridge, runs intermittently.
40-60W
8-12 hrs eq.
Air conditioning (window AC through inverter)
Portable or van-roof AC unit.
800-1200W
4-8 hrs
Laptop charger (through inverter)
15 inch laptop.
60-90W
4 hrs
Microwave (through inverter)
1000W microwave, typical use.
1000W
0.25 hrs
Coffee maker (through inverter)
Drip coffee maker.
800-1000W
0.1 hrs
Shower pump (12V)
Water pressure pump if tank-fed.
80-150W
0.25 hrs
Fan (12V)
Ventilation fan, van circulation.
10-30W
6 hrs
Phone / tablet charger (through inverter)
USB charging via AC adapter.
10-20W
2 hrs
Laptop (direct 12V connector, if available)
Some laptops support 12V input.
60-90W
4 hrs
Figures are approximate for modern appliances. Devices with compressors (fridge, AC) cycle on and off, so listed wattage is peak. Water heaters and cooking appliances draw full power for short periods. For precision, measure your own appliances with a plug-in meter.
Battery chemistry: depth of discharge and usable capacity
The nominal amp-hour rating is not the same as usable capacity. How much you can safely discharge depends on battery chemistry and its lifespan target.
Chemistry
Depth of discharge
Usable per 100Ah
Cycles
Best for
Lead-acid (flooded)
Traditional wet cell, cheapest.
50%
50Ah
500-1000
Budget temporary
AGM sealed lead-acid
No acid spill risk, maintenance-free.
50%
50Ah
500-1000
Marine, RV basic
LiFePO4 (Lithium Iron Phosphate)
Modern standard. Can be left at any SOC.
80-90%
80-90Ah
3000-5000
Van life, off-grid
LiPo (Lithium polymer)
Lighter, but less stable. Needs BMS.
80%
80Ah
1000-2000
Weight-critical
Depth of discharge (DoD) is the safe percentage you can discharge from 100% charge without permanent damage. Lead-acid at over 50% DoD suffers sulfation and life is cut short; LiFePO4 can go to 100% DoD with no damage, but 80-90% is typical to preserve longevity.
Inverter and charge controller sizing
Inverter: Must handle your peak load with headroom for surge current (motor startup). If your heaviest appliance is a 1500W water heater, get a 3000W inverter (1500W x 2x surge factor). Choose pure sine wave for sensitive electronics like laptops.
MPPT charge controller: Sized in amps, based on your panel array and system voltage. A 400W panel array on a 12V system pulls about 30A; get a 40A MPPT controller for headroom. MPPT is more efficient than PWM, especially if your panel voltage is much higher than battery voltage.
The losses shown above (3% MPPT, 10% inverter) are typical for quality equipment. Cheap controllers or inverters can double these losses.
The panel wattage needed depends on your daily energy consumption in watt-hours, how many days of autonomy you want without sun, and the average peak sun hours in your location. To calculate: take your daily watt-hours, divide by peak sun hours to get the basic panel size, then increase it by 30-50% to account for charging the battery bank on cloudy days and maintaining charge during autonomy days. The calculator above shows the exact formula.
What is depth of discharge and why does it matter?
Depth of discharge (DoD) is the percentage of a battery that you can safely discharge without damaging it. Lead-acid batteries should only be discharged to 50%, meaning a 200Ah lead-acid battery gives you only 100Ah of usable capacity. LiFePO4 batteries can be discharged to 80-90% safely, so a 200Ah LiFePO4 battery gives you 160Ah of usable capacity. This is why LiFePO4 batteries provide much better capacity despite being the same amp-hour rating.
What are peak sun hours and how do I find my location's value?
Peak sun hours is the equivalent number of hours per day that solar irradiance averages 1000 watts per square meter. It accounts for the sun's angle changing throughout the day. Most of the southwestern United States gets 5-6 peak sun hours per day in summer, while the northeast gets 3-4. Winter values are typically half of summer. Use your location's annual average or use summer values for a conservative design that works year-round.
Why does the calculator show system losses separately?
Energy is lost at multiple stages: inverter conversion (typically 10% loss), charge controller inefficiency (typically 3-5% loss), and wiring resistance. These losses compound, meaning you need more panel capacity than your calculator might suggest. This calculator breaks down each loss so you can see why a theoretical 1200Wh per day load needs 1500-1800Wh of solar input, not just 1200Wh.
What size battery bank do I need?
Multiply your daily watt-hours by your desired days of autonomy (typically 2-5 for off-grid systems), then divide by your battery chemistry's usable depth of discharge. For example, if you use 1000Wh daily and want 3 days autonomy: 1000 x 3 = 3000Wh needed. For lead-acid (50% DoD), you need 6000Wh total capacity, or 500Ah at 12V. For LiFePO4 (80% DoD), you need 3750Wh capacity, or 312Ah at 12V. The calculator handles this automatically.
Should I choose lead-acid or LiFePO4?
LiFePO4 is now the practical choice for van and off-grid systems. Although it costs more upfront, a single 200Ah LiFePO4 battery provides the usable capacity of 400Ah of AGM lead-acid, weighs much less, and lasts far longer. Lead-acid batteries need to be replaced 3-4 times over a 10-year van life span, while LiFePO4 batteries are rated for 3000-5000 charge cycles. The total cost of ownership favors LiFePO4 unless budget is extremely tight for a short-term setup.